Anaerobic digestion is a promising method for organic waste treatment. While the obtained digestate can function as fertilizer, the liquid fraction produced is rather problematic to discharge due to its high nitrogen and chemical oxygen demand contents. Microalgae have great potential in sustainable nutrient removal from wastewater. This study aimed at evaluating native Swedish microalgae cultivation (batch operation mode, 25°C and continuous light of 80 μmol m−2 s−1) on anaerobic digestion effluent of pulp and paper sludge (PPS) or chicken manure (CKM) to remove ammonium and volatile fatty acids (VFAs). While algal strains, Chlorella vulgaris, Chlorococcum sp., Coelastrella sp., Scotiellopsis reticulata and Desmodesmus sp., could assimilate VFAs as carbon source, acetic acid was the most preferred. Higher algal biomass and cell densities were achieved using PPS compared to CKM. In PPS, Coelastrella sp. and Chlorella vulgaris reached the highest cell densities after 15 days, about 79 × 106 and 43 × 106 cells mL−1, respectively. Although in PPS, ammonium was completely assimilated (195 mg L−1), this was only 46% (172 mg L−1) in CKM. Coelastrella sp. produced the highest biomass concentration independently of the medium (1.84 g L−1 in PPS and 1.99 g L−1 in CKM). This strain is a promising candidate for nutrient removal and biomass production in the aforementioned media, followed by Chlorella vulgaris and Chlorococcum sp. They have great potential to reduce the environmental impact of industrial anaerobic digestion effluents in Nordic countries.

The treatment of organic waste using anaerobic digestion is a promising and well-matured organic waste management method. However, the effluent from anaerobic digestion has a significant discharge risk due to its high ammonium content. Microalgae could be a valuable solution to remove this nitrogen. This work aimed at evaluating the growth of three Nordic microalgae strains (Chlorella vulgaris, Chlorococcum sp. and Coelastrella sp.) in different concentrations of effluent from anaerobic digestion of municipal sewage sludge. None of the strains was able to grow in effluent diluted two times (X2) or three times (X3) due to the high ammonium content (600 and 400 mg L−1, respectively). While Chlorococcum sp. showed a lag phase of 7 and 11-days in 5 times (X5) and 7 times (X7) diluted effluent, respectively, this strain demonstrated 53 % and 86 % total ammonia nitrogen (TAN) removal efficiency after 15 days; in X10 its TAN removal was 100 %. Without any lag phase Coelastrella sp. showed the same TAN removal efficiencies in X5 and X7 as Chlorococcum sp. However, C. vulgaris had the highest TAN removal in X5 (90%) and X7 (90%). Furthermore, this strain showed the highest amount of biomass dry weight production in all media (1.1 g L−1 in X5). Therefore, C. vulgaris and Chlorococcum sp. are promising candidates for nitrogen removal and sustainable algae biomass production, resulting in mitigating the environmental issues of anaerobic digestion effluents in Nordic countries through the conversion of waste streams into resources.